posts_where filter using custom fields for dates

Currently I have the loop listing posts between two dates using the posts_where filter –

function filter_where( $where = '' ) {
		$where .= " AND post_date >= '2000-01-01' AND post_date <= 2004-12-31' ";
		return where;
	}
	add_filter('posts_where', 'filter_where');
	query_posts($query_string);
	while (have_posts()) :
      	the_post();
      	the_content();
	endwhile;

In a meta_key called ‘original_date’ I have stored a different date for each post that I wish to use instead of the post_date. How do I correctly call up this meta_key into the $where query to fulfil the same purpose as the post_date?

$where .= " AND [meta_key=original_date] >= '2000-01-01' AND [meta_key=original_date] <= '2004-12-31' ";

For example, the following SQL query doesn’t seem to work, even though correct values (2001-10-29, 2004-11-03, etc.) are present in the ‘original_date’ meta_key of the posts –

global $wpdb;
	$where .= " AND (SELECT meta_value FROM $wpdb->postmeta WHERE meta_key='original_date' ) >= '2000-01-01' AND (SELECT meta_value FROM $wpdb->postmeta WHERE meta_key='original_date' ) <= '2004-12-31' ";

whereas the following works fine on the same posts using their post_date as reference –

$where .= " AND post_date >= '2000-01-01' AND post_date <= 2005-12-31' ";

Perhaps the meta_value array needs to be stripped of some extraneous material in order to format the results into the same form as post_date? How to go about this?